This invention relates light-emitting devices, for example devices suitable as display devices.
One specific class of display devices is those that use an organic material for light emission. Light-emitting organic materials are described in PCT/WO90/13148 and U.S. Pat. No. 4,539,507, the contents of both of which are incorporated herein by reference. The basic structure of these devices is a light-emitting organic layer, for instance a film of a poly(p-phenylenevinylene (xe2x80x9cPPVxe2x80x9d), sandwiched between two electrodes. One of the electrodes (the cathode) injects negative charge carriers (electrons) and the other electrode (the anode) injects positive charge carriers (holes). The electrons and holes combine in the organic layer generating photons. In PCT/WO90/13148 the organic light-emitting material is a polymer. In U.S. Pat. No. 4,539,507 the organic light-emitting material is of the class known as small molecule materials, such as (8-hydroxyquinoline)aluminium (xe2x80x9cAlq3xe2x80x9d). In a practical device one of the electrodes is typically transparent, to allow the photons to escape the device.
FIG. 1 shows the typical cross-sectional structure of an organic light-emitting device (xe2x80x9cOLEDxe2x80x9d). The OLED is typically fabricated on a glass or plastic substrate 1 coated with a transparent anode electrode 2 of a material such as indium-tin-oxide (xe2x80x9cITOxe2x80x9d) that is suitable for injecting positive charge carriers. Such coated substrates are commercially available. This ITO-coated substrate is covered with at least a layer of a thin film of an electroluminescent organic material 3 and a final layer forming a cathode electrode 4 of a material that is suitable for injecting negative charge carriers. The cathode electrode is typically of a metal or alloy. Other layers can be included in the device, for example to improve charge transport between the electrodes and the electroluminescent material.
FIG. 2 shows the energy levels of the layers in the device of FIG. 1. Under forward bias it is feasible for holes to pass from the anode electrode 2 and for electrons to pass from the cathode electrode 4 into the emitting layer, where they can combine. Under reverse bias it is not favourable for electrons to pass from the electrode 2 into the emitting layer, or for holes to pass from the electrode 4 into the emitting layer 2. The device thus behaves as a diode.
Important measures of the performance of OLEDs are lifetime, power efficiency and turn-on voltage. It has been recognised that the lifetime of a typical OLED can often be extended by driving it intermittently or even by applying temporary reverse voltages between the anode and cathode electrodes (AC driving). However, since no light is emitted when there a negative applied voltage an AC drive scheme reduces light output from the OLED unless the device is driven harder during the times when it is under forward bias. This harder driving can a accelerate degradation of the device. In some circumstances this can significantly offset any gains in lifetime from intermittent or AC driving.
According to the present invention there is provided an electroluminescent device comprising: a first electrode; a second electrode; and a light-emissive region of electroluminescent organic material between the electrodes; and wherein the first electrode comprises a first material capable of injecting positive charge carriers into the light-emissive region and a second material capable of injecting negative charge carriers into the light-emissive region; and the second electrode comprises a third material capable of injecting positive charge carriers into the light-emissive region and a fourth material capable of injecting negative charge carriers into the light-emissive region.
Preferably the device is capable of emitting light from the light-emissive region when the said negative charge carriers are injected from the first electrode and positive charge carriers are injected from the second electrode and is capable of emitting light from the light-emissive region when the said positive charge carriers are injected from the first electrode and negative charge carriers are injected from the second electrode.
Preferably the first, second third and fourth materials are capable of injecting charge carriers as said above when a voltage of magnitude less than 20, 10 or 5V is applied between the electrodes. One or both of the first and third materials may have a work function above 4.0 eV or 4.5 eV. One or both of the second and fourth materials may have a work function below 3.5 eV or 3.0 eV.
The first electrode suitably has a surface facing the region of electroluminescent material at which the first material and the second material are present. Preferably regions of the first and second materials are located adjacent to the region of electroluminescent material. The second electrode suitably has a surface facing the region of electroluminescent material at which the third material and the fourth material are present. Preferably regions of the third and fourth materials are located adjacent to the region of electroluminescent material.
The first and/or third material may, for example, be gold or platinum or ITO. The first and third materials may be the same or different. The second and/or fourth material may, for example, be an alkali metal or and alkali earth metal or an oxide or fluoride of an alkali metal or an alkali earth metal, suitably having a low work functionxe2x80x94for example below 3.5 eV. The second and/or fourth material may be a fluoride or oxide of a low work function metal such as Li, Ca, Mg, Cs, Ba, Yb, Sm etc. The second and/or fourth materials may be the same or different.
Preferably either or both of the electrodes may be light-transmissive, most preferably transparent.
According to a second aspect of the present invention there is provided a method of driving an electroluminescent device as described above, comprising applying an alternating current drive scheme to the electrodes. The alternating current drive scheme may comprise repeatedly biasing the first electrode positively relative to the second electrode and subsequently biasing the first electrode negatively relative to the second electrode. The scheme may or may not be periodic. The scheme may or may not include periods when neither electrode is biased relative to the other.
Where the device is driven by a scheme that includes periods of opposite biasing it is preferred that the voltage applied across the electrodes when the first electrode is biased positively relative to the second electrode is such as to cause electron/hole recombination in a different zone of the light-emissive region than does the voltage applied across the electrodes when the first electrode is biased negatively relative to the second electrode.
The light-emitting material is suitably an organic material and preferably a polymer material. The light-emitting material is preferably a semiconductive and/or conjugated polymer material. Alternatively the light-emitting material could be of other types, for example sublimed small molecule films or inorganic light-emitting material. The or each organic light-emitting material may comprise one or more individual organic materials, suitably polymers, preferably fully or partially conjugated polymers. Example materials include one or more of the following in any combination: poly(p-phenylenevinylene) (xe2x80x9cPPVxe2x80x9d), poly(2-methoxy-5(2xe2x80x2-ethyl)hexyloxyphenylenevinylene) (xe2x80x9cMEH-PPVxe2x80x9d), one or more PPV-derivatives (e.g. di-alkoxy or di-alkyl derivatives), polyfluorenes and/or co-polymers incorporating polyfluorene segments, PPVs and related co-polymers, poly(2,7-(9,9-di-n-octylfluorene)-(1,4-phenylene-(4-secbutylphenyl)imino)-1,4-phenylene)) (xe2x80x9cTFBxe2x80x9d), poly(2,7-(9,9-di-n-octylfluorene)-(1,4-phenylene-((4-methylphenyl)imino)-1,4-phenylene-((4-methylphenyl)imino)-1,4-phenylene)) (xe2x80x9cPFMxe2x80x9d), poly(2,7-(9.9-di-n-octylfluorene)-(1,4-phenylene((4-methoxyphenyl)imino)-1,4-pherylene-((4-methoxyphenyl)imino)-1,4-phenylene)) (xe2x80x9cPFMOxe2x80x9d), poly (2,7-(9,9-di-n-octylfluorene) (xe2x80x9cF8xe2x80x9d) or (2,7-(9,9-di-n-octylfluorene)-3,6-Benzothiadiazole) (xe2x80x9cF8BTxe2x80x9d). Alterative materials include small molecule materials such as. Alq3. The light-emitting region may include two or more such materials.
One or more charge-transport layers may be provided between the light-emitting region and one or both of the electrodes, or integrated into the light-emitting region. The or each charge transport layer may suitably comprise one or more polymers such as polystyrene sulphonic acid doped polyethylene dioxythiophene (xe2x80x9cPEDOT-PSSxe2x80x9d), poly(2,7-(9,9-di-n-octylfluorene)-(1,4-phenylene-(4-imino(benzoic acid))-1,4-phenylene-(4-imino(benzoic acid))-1,4-phenylene)) (xe2x80x9cBFAxe2x80x9d), polyaniline and PPV.